1. Field of the Invention
The present invention relates to a fixing apparatus for fixing an unfixed toner image onto a recording medium.
2. Description of the Related Art
In an electrophotographic image forming apparatus, there is provided a fixing apparatus for fixing an unfixed toner image formed on a recording medium, such as paper, onto the recording medium by heating and pressurizing the unfixed toner image. One form of the fixing apparatus encompasses a fixing apparatus including a fixing roller having a rubber layer, for conveying a recording medium while heating the recording medium, a pressure unit for forming a fixing nip together with the fixing roller, and a heating unit for heating the fixing roller by forming a heating nip together with the fixing roller (Japanese Patent Application Laid-Open No. 2011-133502). In order to fix the unfixed toner image onto the recording medium, it is necessary to apply a predetermined amount of heat or more with respect to the recording medium and the unfixed toner. This amount of heat is determined based on the temperature of the heating unit, the time period for transmitting heat from the heating unit to the fixing roller, and the time period for applying heat from the fixing roller to the toner. Note that, the time period for transmitting heat from the heating unit to the fixing roller significantly affects the nip width of the heating nip. Further, the time period for applying heat from the fixing roller to the toner significantly affects the nip width of the fixing nip. Here, when the temperature of the heating unit is increased, the amount of heat to be applied to the unfixed toner is increased, but there is a limit to the increase in temperature from the viewpoint of the heat-resistant temperature of various members forming the fixing apparatus or the image forming apparatus, the temperature increase in the apparatus, energy saving, and the like.
Accordingly, in order to fix the unfixed toner image onto the recording medium satisfactorily, it is necessary to set the nip width of each of the fixing nip and the heating nip to be a predetermined width or more. Therefore, it is necessary to set the hardness of the fixing roller and the pressure force of the heating unit and the pressure unit with respect to the fixing roller so that the nip width reaches a predetermined width or more.
However, when the image forming apparatus has been used for a long period of time, and the image forming apparatus is approaching the end of its life, the durability of the rubber layer of the fixing roller is degraded to decrease the hardness thereof. Along with the decrease in hardness, the compression deformation amount of the rubber layer of the fixing roller increases. As a result, the torque required for driving the fixing roller increases, and the risk of damage to the fixing roller also increases. Further, when the nip width is enlarged and the heat supply to the recording medium becomes excessive, the recording medium is deformed significantly, and the stacking property of the recording medium having an image formed thereon is also degraded.
Thus, in order to solve various problems to be caused when the nip width is too large while realizing satisfactory fixing property, it is necessary to regulate two nip widths within a predetermined range while setting the two nip widths to be a predetermined width or more.